It has been generally recognized that semiconductor diode devices which have substantial depth beyond that readily achievable in the planar geometry presently in general use would offer important advantages to the user. Thus, shallow diode arrays produced on the surface of thin wafer silicon by diffusion or expitaxial techniques and used for imaging are not capable of use in the X-ray and infrared radiation regions because of the low absorption constant of silicon for such radiation. While a thick target could compensate for this shortcoming of silicon, resolution in the shallow diode array geometry would be lost by diffusion and smearing out of electrons and holes parallel to the target surface before reaching the shallow surface diodes from generation points within the target bulk. But if use could be made of deep diode geometry, both high sensitivity (because of target thickness) and good resolution (because of the detecting diodes deep within the target bulk) might be achieved.
Efforts in the prior art to produce deep diode arrays have not been successful enough for general application. These efforts are best exemplified by the disclosure in U.S. Pat. 2,813,048, issued Nov. 12, 1957 to W. G. Pfann. Apparently, a principal difficulty encountered by Pfann and those following Pfann's teachings is the fact that the straight-line deep diodes shown in this patent were not obtainable in actual practice. Instead, Pfann's P-N junctions were curved as droplets followed by a banding course in migration through the semiconductor wafer bodies. Even in relatively thin sections used by Pfann and others at that time, the curving course of droplet migration was apparent, and it is not possible to compensate for this departure from the idealized form of deep diode illustrated and described in the Pfann patent.